In a magnetron 90 disclosed in Patent Document 1, a metal sealing body 80 (hereinafter called a metal sleeve 80) is joined to an anode cylinder. FIG. 5 is a diagram showing the metal sleeve 80 of the magnetron 90 in related art. As shown in FIG. 5, the metal sleeve 80 includes a flange part 81 contacting a pole piece 84 (hereinafter called a pole piece 84), a cylinder part 82 continuous with the flange part 81, and a folded-back part 83 which continues from the cylinder part 82 and which is folded back inside the tube.
The cylinder part 82 includes a concentric first cylinder part 82a, a second cylinder part 82b having the same central axis as the central axis of the metal sleeve 80, and a taper part 82c continuous with the first cylinder part 82a and the second cylinder part 82b. 
The first cylinder part 82a continues with the second cylinder part 82b through the taper part 82c in which an inside diameter changes gradually. An inside diameter φ1 of the first cylinder part 82a is smaller than an inside diameter φ2 of the second cylinder part 82b. Also, the inside diameter φ2 of the second cylinder part 82b of the metal sleeve 80 is formed in about the same dimension as an inside diameter D of a falling part of the pole piece 84.
Patent Document 1: JP-A-2005-050572